This invention relates to the administration of a therapeutic gas such as nitric oxide (NO) to patients for therapeutic effect. In particular, it relates to a system wherein a controlled, predetermined supplemental volume of a dose of NO is provided to the patient with each inhalation by the patient.
The function of the administration of NO has been fairly widely published and typical articles appeared in The Lancet, Vol. 340, October 1992 at pages 818-820 entitled "Inhaled Nitric Oxide in Persistent Pulmonary Hypertension of the Newborn" and "Low-dose Inhalational Nitric Oxide in Persistent Pulmonary Hypertension of the Newborn" and in Anesthesiology, Vol. 78, pgs. 413-416 (1993), entitled "Inhaled NO-the past, the present and the future".
The actual administration of NO is generally carried out by its introduction into the patient as a gas and commercially available supplies are provided in cylinders under pressure and may be at pressures of about 2000 psi and consist of a predetermined mixture of NO in a carrier gas such as nitrogen. A pressure regulator is therefore used to reduce the pressure of the supply cylinder to working levels for introduction to a patient.
The concentration administered to a patient will vary according to the patient and the need for the therapy but will generally include concentrations at or lower than 100 ppm. There is, of course, a need for that concentration to be precisely metered to the patient since an excess of NO can be harmful to the patient.
Various delivery devices have been used or suggested that respond to the patient attempting to inhale to deliver a pulsed dose of NO to the patient and such pulsing devices have also been shown to have therapeutic effect on the patient, for example, as described in Higenbottam PCT patent application WO 95/10315 and the publication of Channick et al "Pulsed delivery of inhaled nitric oxide to patients with primary pulmonary hypertension", Chest/109/ June 1996. In such pulsatile dosing devices, a pulse of NO is administered to the patient as the patient inhales spontaneously.
One difficulty of the present pulsed NO devices is that the concentration of the NO used by the systems, and which is commercially supplied in pressurized cylinders mixed in nitrogen, is relatively low since high concentrations of NO are injurious to patients. Therefore the devices use NO gas mixed in nitrogen with a concentration of NO of about 100 ppm. Thus, large supplies of the NO containing therapy gas cylinders are required to treat the patient rather than a lesser quantity of a high concentration of NO in nitrogen of about 800 ppm which can then be diluted down to about 100 ppm to be able to administer the proper dosage to the patient. In addition if all of the balance gas for NO is nitrogen, the amount of non-oxygenated gas that the patient receives per breath is increased and therefore, the potential of delivering a hypoxic breath exists.
There are certain devices that carry out the blending of the NO containing gas with a diluent or mixing gas and one such system is shown and described in Wessel et al, Delivery and Monitoring of Inhaled NO in patients with pulmonary hypertension (Critical Care Medicine, Vol. 22, No. 6 1995). In the Wessel et al system, however, the medical NO is blended with nitrogen through the use of medical blenders that require continuous flows of all the gases to properly blend the gases together prior to administration to the patient or to a ventilator.
In another system, that of Briend et al, U.S. Pat. No. 5,651,358, pulsed injections of NO are administered into a ventilator circuit using a delivery device that is triggered by a signal from the respirator.
A further device for the pulsed administration of NO has been developed and which is described in copending U.S. Ser. No. 08/857,924 filed May 16, 1997 and the disclosure of which is incorporated herein by reference. In that further system, the actual pulse of NO is corrected for ambient conditions to assure that the dose of the NO pulse is precise and does not vary depending upon differing ambient conditions.
Accordingly it would be desirable to provide a pulsing device that enables one to use a patient initiated breath that would administer a safe, diluted therapeutic mixture of NO during the patient's inhalation yet be able to use, as the supply of NO, a relatively high concentration of NO in a gas such as nitrogen and the NO mixed with a diluent gas on a pulse to pulse basis.